This invention relates to the field of reactors that are intended to carry out non-catalytic partial oxidation reactions starting from very diverse hydrocarbon fractions for the purpose of producing a mixture of carbon monoxide (CO) and hydrogen (H2) that is called synthesis gas. The reactors that are targeted by this invention are more particularly intended for small-capacity applications as opposed to standard industrial applications such as Fischer-Tropsch synthesis or ammonia synthesis.
In this case, installed thermal powers of between 0.1 kW and 1000 kW are considered. The targeted markets are then the supply of hydrogen-rich gas for fuel-cell-type (PAC) applications or hydrogen enrichment of thermal engines.
In the text below, this type of reactor will be called a POX reactor, the usual abbreviation of partial oxidation reactions.
The use of a partial oxidation reactor for carrying out the generation of synthesis gas, and more particularly an H2-rich synthesis gas, is not original in itself. By contrast, however, the technology of the reactor that is used may be capable of novelty in a field where for low power levels, the majority of the players are oriented toward catalytic and non-thermal POX concepts so as to avoid having to manage temperatures higher than 900 or 1000° C.
In some cases, the strongly exothermic partial oxidation reactions are followed by endothermic vaporeforming reactions, whereby the introduction of water vapor may take place in the form of partial oxidation reactions or in the form of vaporeforming reactions. All of the partial oxidation and vaporeforming reactions, when they take place simultaneously in the same reaction chamber, are then designated under the name of an autothermal process (ATR abbreviation).
This invention relates to the technology of the reactor for implementation of the non-catalytic POX reactions and also applies when these non-catalytic POX reactions are followed by catalytic vaporeforming reactions, whereby the two types of reactions take place in separate reaction chambers.
The technology of the POX reactors has been experiencing a renewal of interest for several years in connection with the production of H2 for the purpose of supplying a fuel cell (PAC). This interest for a suitable POX reactor technology is quite often encountered within the context of on-board reactors when the PAC is intended to provide electric energy for the motorization of a vehicle.
Below, we are providing a general picture of recent developments in the technology of POX reactors in the non-catalytic domain:
In 2000, OEL-WARME Institut published two articles on the design of a partial oxidation reactor developed at the Université d'Aix la Chapelle. In this reactor, the chamber is essentially divided into two parts: a first part termed the cold flame zone in which the hydrocarbons are mixed with preheated air so as to obtain a controlled oxidation reaction at a temperature of between 310° C. and 480° C., now called cold flame, and a second part that constitutes the core of the POX with a temperature of higher than 1000° C.
During the start-up, the hydrocarbons are vaporized in the preheated air to start up the cold flame. Then, the air temperature is lowered to obtain conditions for stabilization of the cold flame in the first part of the reactor where the temperature is kept lower than 480° C. because beyond this temperature, the reaction goes out of control by ignition of the fuel-air pre-mixture and transition from the cold-flame state to the standard combustion state.
The adiabatic combustion temperature is then essentially reached. The start-up of the POX section is carried out by standard ignition. The authors believe that the cold flame offers a determining advantage in that it would be responsible for the low level of soot that is observed experimentally.
This concept may very likely lead to a limitation of the soot production because of the premixing and the oxidation of heavy molecules of the fuel that are produced in the cold flame, but it imposes very significant limitations on the preheating temperature of the air and the fuel. Actually, beyond 480° C. in the cold flame and therefore from a preheating of the air-fuel mixture that is higher than about 350° C., there is a risk of losing control of the reaction and a return of flame into the cold flame chamber.
This limitation of the preheating produces a very significant economic penalty because a large fraction of the fuel is then to be oxidized to reach the reaction temperature, which greatly penalizes the yield of the generator relative to a system where it would be possible to preheat the air to more than 1000° C. before the input into the combustion zone.
In the reactor concept according to the invention, it is also sought to limit the formation of the soot but by removing the constraint on the preheating of the air and the feedstock. To do this, it was chosen to optimize the hydrodynamics of the reactor by dividing the reaction zone where the POX reaction is carried out into a first perfect-mixing reaction zone followed by a second piston reaction zone with or without a staged injection of oxidant.
U.S. Pat. No. 3,516,807 of June 1970 of Texas Instruments refers to an integrated POX reactor in which is carried out the preheating of air entering via the combustion effluents with the use of the fuel injector as a Venturi tube being used to draw in the combustion air as a particular feature. This design element of the injector is repeated in the claims. The importance of the thermal integration on the yield is not noted; by contrast, it is clearly indicated in the text that the reactor should operate at a temperature of 1200° C. or more to lead to reasonable dwell times, and therefore it is imperative to preheat the air that enters with the combustion gases.
Relative to this patent, this invention is based on a considerably more advanced thermal integration in which is considered the possibility of reaching chamber temperatures that are higher than 1300° C. and even 1500° C. to limit both the dwell times and the soot formation in the partial oxidation chamber.
Likewise, for the reduction of soot and non-burned methane or non-methane residues, it is very important to consider the hydrodynamics of the POX reactor so as to carry out the combination of a first reaction zone with an essentially isothermal perfect mixing flow and a second reaction zone with a piston flow that is also essentially isothermal, at least over a portion of its length.
In a publication by P. Marty; M. Falempe; and D. Grouset entitled “The Use of Semi-Detailed Kinetic Diagrams for a Study of the Influence of Temperature in the Reforming of Fuels Without a Catalyst,” presented at the Belfort Conference in November 2000, note is taken of a reactor that is improperly called an autothermal reactor (ATR) because of the heat recovery carried out on the combustion gases. Actually, it is possible to derive from information contained in said publication that the concern for thermal integration was duly taken into account by the author for designing a reactor that operates at a short dwell time, but there is no information on the technology to use for optimizing thermal transfers and heat recovery and to carry out flows so as to limit soot formation.
Patent WO 96/36836 describes a staged combustion system with integrated preheating, i.e., a heat exchange between the combustion gases and the combustion air. This patent essentially describes a method for reducing the NOX that makes use of two combustion chambers.
Patents EP 0 255 748 B, EP 0 291 111 B and U.S. Pat. No. 5,653,916 describe a non-catalytic POX reactor that has a burner technology that consists of at least 4 concentric tubes that are alternately supplied by an oxidizing gas that contains oxygen and by a hydrocarbon-rich gas. The momentum that is necessary to the mixing is essentially created by the injection speed of the hydrocarbon that is between 50 and 150 m/s.
Patent EP 0 380 988 B describes a partial oxidation reactor that uses an injector that consists of 3 concentric tubes. The central tube makes it possible to inject water vapor or CO2 at a supersonic speed at the mixing point of the combustion air and the fuel.
This injection makes it possible to obtain a very quick mixing and therefore in principle to limit the soot formation.
U.S. Pat. No. 5,958,297 proposes a non-catalytic POX reactor technology that is also applicable for the POX section of ATR reactors. The operating temperatures are between 1000 and 1500° C. for the POX and between 900° C. and 1400° C. for the ATR.
A reduction of the alumina contained in the refractory materials (containing about 90% alumina) into volatile aluminum oxides was observed at these temperature levels that correspond to a reducing atmosphere in the reactor.
To eliminate this problem, the cited patent teaches carrying out on the wall of the reactor or behind said wall an endothermic vaporeforming reaction by circulation of a portion of the gases that have not reacted upon contact with said wall, whereby the latter was made catalytic. The vaporeforming reaction employed makes it possible to lower the wall temperature by 100° C. to 300° C. and therefore to limit the reduction of alumina.
Patent US 2001/0009732 relates to the integration of a POX reactor with a fuel cell (PAC) of solid-oxide type (SOFC). The diagram of the process exhibits a high degree of integration between the hydrogen generator and the SOFC cell, whereby the effluents of the cell are used to preheat the combustion air of the POX.
A zone for recirculation of the combustion gases is located at the fuel injection site so as to homogenize the temperatures and to limit the formation of soot. The integration of the POX with the cell itself requires that the POX operate at temperature levels that cannot exceed 1000° C. At this temperature, the reaction times are relatively significant, on the order of several seconds, and there is a risk of obtaining relatively large methane concentrations in the effluents of the POX.